Layers of ferro-electric materials can be used to store data on a surface. Ferro-electric crystals are known to have a bipolar electrical moment even in the absence of an external electric field.
One way to create an array of data storage elements on a layer of ferro-electric material is to modify locally the ferro-electric characteristics of the material. The direction of the bipolar moment can be modified by applying an external electrostatic field.
Such modification can be achieved by locally applying an electrostatic field between a near field microscope tip positioned on one side of the layer and a plane counter electrode situated on the opposite side of the layer and parallel thereto.
It has been shown by Cho et al. (Cho et al; Nanotechnology, 17, 006, S137-S141) that a 10 V electrostatic field applied for 500 ps modifies the direction of polarization of an 18 nm thick ferro-electric crystal mounted on a chromium counter electrode.
The volumes of ferro-electric crystal whose ferro-electric nature is modified are referred to as nanodomains. The smallest nanodomain diameter (measured parallel to the layer) obtained experimentally by Cho et al. is 5.1 nm. These authors also prepared arrays of nanodomains on a layer. The smallest pitch obtained experimentally in a nanodomain array is 8.0 nm.
The polarization of each nanodomain constitutes one item of information, typically binary information. Generally, the smaller the nanodomains, the greater the density of the information that can be stored on a ferro-electric layer.
For example, it can be calculated that, if the ferro-electric properties are modified with an accuracy of the order of 5 nm, a memory can be obtained with a density of 10 Tbit per square inch (approximately 6.5 cm2).
The area of the nanodomains on the surface of the ferro-electric layer is known to be directly proportional to the thickness of the ferro-electric layer.
The Smart Cut™ technology (see for example U.S. Pat. No. 5,374,564) can advantageously be used to obtain a thin ferro-electric layer. See for example U.S. Pat. No. 6,607,969, which describes the use of thin ferro-electric layers.
Transfer techniques including grinding and polishing steps can also be used. On this topic, see the paper by Cho et al. cited above.
In practice, the radii of curvature of near field microscope tips (for example those of an atomic force microscope (AFM)) are of the order of 10 nm or greater, which constitutes an important limiting factor. These points are conductive, for example covered with metal.
Note that according to the near field microscopy method used, the near field microscope tip can be in contact or not with the surface at the moment of application of the electrostatic field.
Moving the point toward the surface reduces the size of the nanodomains, although with a risk of degrading the surface.
In any event, it is difficult for the moment to obtain nanodomains with a size less than 5 nm, in particular because of the size of the microscope tips.
Moreover, it has been found that existing methods offer limited reproducibility in the formation of small nanodomains and the modification of their ferro-electric properties.
Two near field microscope tips are never completely identical and it is found that, from an operation of writing with a given point to another operation of writing with another point, the size and geometry of the resulting nanodomains vary.